TC str2gr conversion not in cache + adapt solve_TC wrt coq hook

[FissoreD]

adapt solve_TC wrt coq hook

• a failing search should raise Not_found exn to be correctly managed by coq
• only the sub_goals should be added to the shelf (cf this)
• apps/tc/tests/lemma_with_max_impl.v test the correct behaviour of this change

str2gr

• the conversion str2gr should be done outside the caching function (cf this) otherwise the error Global reference not found: XXX is raised.
• Files tests/test_import/{f1 and f2}.v verify that now we have the right behavior
• I add the module OSet in the takeover for the set of overridden modes: this ease modification of code